[RFC v2 1/7] vhost: expose used buffers

@@ -2376,10 +2376,20 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
 	vring_used_elem_t __user *used;
 	u16 old, new;
 	int start;
+	int copy_n = count;
 
+	/**
+	 * If in order feature negotiated, devices can notify the use of a batch of buffers to
+	 * the driver by only writing out a single used ring entry with the id corresponding
+	 * to the head entry of the descriptor chain describing the last buffer in the batch.
+	 */
+	if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
+		copy_n = 1;
+		heads = &heads[count - 1];
+	}
 	start = vq->last_used_idx & (vq->num - 1);
 	used = vq->used->ring + start;
-	if (vhost_put_used(vq, heads, start, count)) {
+	if (vhost_put_used(vq, heads, start, copy_n)) {
 		vq_err(vq, "Failed to write used");
 		return -EFAULT;
 	}

@@ -2410,7 +2420,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
 
 	start = vq->last_used_idx & (vq->num - 1);
 	n = vq->num - start;
-	if (n < count) {
+	if (n < count && !vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
 		r = __vhost_add_used_n(vq, heads, n);
 		if (r < 0)
 			return r;

@@ -236,6 +236,7 @@ enum {
 	VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
 			 (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
 			 (1ULL << VIRTIO_RING_F_EVENT_IDX) |
+			 (1ULL << VIRTIO_F_IN_ORDER) |
 			 (1ULL << VHOST_F_LOG_ALL) |
 			 (1ULL << VIRTIO_F_ANY_LAYOUT) |
 			 (1ULL << VIRTIO_F_VERSION_1)